1. Field of the Invention
The present invention relates in general to the provision of photoetched grooves or slots between the faying surfaces of a diffusion bond and relates in particular to the formation of fluid flow channels in a diffusion bonding foil located between the faying surfaces of an airfoil blade or vane of a gas turbine engine.
2. Description of Prior Developments
Diffusion bonding, which includes diffusion brazing and diffusion welding, is a form of solid state welding which is accomplished by bringing the surfaces of two or more members to be joined together under moderate pressure and elevated temperature. Diffusion bonding usually is carried out in a controlled atmosphere wherein a coalescence of the interfaces or faying surfaces can occur. Melting or fusion of the members being bonded is not generally associated with this process.
Diffusion bonding typically requires weld durations ranging from minutes to hours. Coalescence of the faying surfaces is produced with minimum macroscopic deformation by diffusion-controlled processes that are induced by applying heat and pressure for a predetermined time interval. In most cases, the equipment used to form diffusion bonds or welds is custom built and welding is carried out in a vacuum, in an inert gas or, in a reducing atmosphere.
A diffusion bond or weld is formed when the faying surfaces of two or more materials are brought together sufficiently close so that short range interatomic forces operate. Initial contact is made between surface asperities when a load is applied. Further contact is made by plastic yielding and creep deformation. Diffusion bonding is usually carried out at a welding temperature equal to or greater than one-half of the melting temperature of the material being bonded, welded or brazed. The original faying surfaces eventually disappear thereby resulting in a completed weld.
Interlayers of other similar metals in the form of thin foils or coatings have been used between faying surfaces to overcome metallurgical bonding problems and/or to facilitate bonding. Interlayers can reduce bonding or welding temperatures, restrict high deformation to the interlayer zone, improve mating of rough surfaces, reduce bonding time, and reduce oxidation. Interlayers should be metallurgically compatible with the base metals and should not form a low toughness zone. In some cases, interlayers having melting points lower than the base metals being welded are used to obtain low welding temperatures.
One application of diffusion bonding is disclosed in U.S. Pat. No. 3,656,222 which is incorporated herein by reference. This patent discloses an airfoil blade blank which is formed by diffusion brazing two separate blade blank portions together using an interlayer sheet of brazing material. Cooling air passages or slots are formed in one or both of the blade blank portions as well as through complementary or matching portions of the interlayer sheet of brazing material. The slotted blade blank portions and interlayers, once diffusion brazed, are used to form a finished airfoil-shaped blade by any suitable method.
Although the finished blade of U.S. Pat. No. 3,656,222 may function suitably for its intended purpose, a continuing need exists for higher operating temperatures in gas turbine engines within which such blades operate. This need may, in part, be satisfied by improving the air cooling rate of these blades by optimizing and increasing the cooling air coverage or surface areas cooled. Such coverage is presently limited by the machining or forming techniques used to cut or form the cooling passages through which cooling air flows to extract the heat from the airfoils heated by hot flowing exhaust gasses.
That is, the more area covered by the cooling slots or passages, the greater the potential for improved cooling performance. Currently used machining techniques are limited as to the size, spacing and location of the cooling slots or passages which may be produced in airfoil blades as well as in any other heated members which may be cooled by internally channeled cooling fluids. Moreover, such conventional machining techniques used to form such cooling slots are quite expensive, time consuming and labor intensive.
Accordingly, a need exists for a method of forming extremely small, narrow, accurately placed cooling grooves or slots within a heated element capable of being cooled by internally flowing cooling fluid. This need is particularly acute in the case of airfoil blades and vanes, particularly along their hot trailing edge portions which tend to experience the highest operating temperatures. Such a method should be relatively easy to perform and result in cost savings over conventional methods.